I would say the major reasons are down to the fact the car is old, the fuel costs are high and no one has any money.
You also have to think because of the reasons above the Supra would no longer fit the daily driver category now (it barely did anyway) but really only a weekend car.
Not a lot of people have money to spend on a car they are only going to use at the weekend, not these days.
It also does not help that Toyota ended stopped production of the Supra with no announcement of a successor which killed the name dead.
Manufactures like Nissan still kept the "Skyline" brand alive to remind the public of the iconic cars.
After the R34 production ended, Nissan still had a huge presence by using the R34 in racing, then not many years later they brought out the Nissan Skyline (which was a bigger version of the 350z) which while it was hideous still kept the “skyline” brand alive, which set it up the huge GTR (R35) release.
Could it be that people have just simply forgotten about the Supra because there has been no effort expended on Toyota’s part to keep the name alive?
I went to a number of Pistonheads meets in my old Supra and found that quite a few people were asking what it was and when reminded they always said “oh yeah I remember, good cars those”.
I think it’s a culmination of Age, Fuel costs, No money and an abandoned brand, so it’s just simply dropped off people’s radar,
It’s been a long time since Toyota have done a descent sports car, so Toyota is not a brand people think of.
Maybe the GT86 will start to bring people back to the Supra… who knows, only time will tell.